<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<input type="text" id="numId" />
		<input type="button" value="阶乘" onclick="cal()" />
		=
		<span id="resultId"></span>
		
		<script type="text/javascript">
			function cal() {
				var num = document.getElementById('numId').value;
				var resultSpan = document.getElementById('resultId');
				// NaN:Not a Number   isNaN()
				if(isNaN(num) || num == '') {
					resultSpan.innerHTML = '<span style="color: red;">您输入的不是一个数字</span>';
					return;
				}
				
				if(num < 0) {
					resultSpan.innerHTML = '<span style="color: red;">负数没有阶乘</span>';
					return;
				}
				
				var result = 1;
				for (var i = num; i >= 1; i--) {
					result = result * i;
				}
				resultSpan.innerText = result;
			}
		</script>
	</body>
</html>
